Heating circuit and method



Nov. 10, 1953 RENDEL 2,658,982

HEATING CIRCUIT AND METHOD Filed Feb. 19, 1952 l CHOKE (0 9155.

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Patented Nov. 10, 1953 BEA-TIN GeGIRCU IT ANDs ME T-HOD r George H. Rendel', Pittsburgh, Pa'., assignor to United States Steel- Corporation, wcorporation ofNew Jersey Application February 19, 1952, Serial No. 272,472

30laims; 1.

This invention relates to improyedcircuitsand methods for electric resistance heating of metal in continuous. processing-lines.

Although not restricted" to any specificmetal product, the invention is especially applicable'tov heating tin coated steel strip in electrolytic tinning lines for the purpose of melting the coating and thereby-improving its properties. Commonlysuchstripisheated bypassing a highamperage alternating current through a portion thereof; the current beingintroduced at two spaced apart conductor rolls around which the strip travels; The power or wattagerequired to produce a given temperature rise in the strip is directly proportional to; the stripspeed, and the current andzvoltage-are proportional to the square root-of this speed. If the density of the current transferred between. a conductor roll and the strip becomes too high at any" one spot, there is a tendency for arcing to occur between the strip and the conductor roll and to damage the coating; Therefore the operating speed has been limited by the'dens-ity of current that can be introduced to the strip without causing arcing. Suchlines must be equipped with means for preventing current from'flowing through an' alter native path through ground which path is parallel to that" through the; strip in the heating zone; In practice; such means'usuailly includes choke cores through which-the strip travels out=- side the conductor roll'sand which creates; high impedance in the strip and; thus" in eiiect break or reduce the alternativecurrent' path: Of course the same effect could be obtaincdby' e1ectrically insulating'the' entire strip from ground, although this latter'practice is avoided for-reasons of safety;

An object" of the present inventionisto" pro-. videimproved heating circuits and methods whichreduce the density of" current which flows between the rolls and thecontinuous metalproduct at anyone place w-ithoutreducing the. total current or-power input, thereby enabling processing" lines to" operate at higher speeds without arcing.

A furtherph-jectdsto' provide improved heating circuits andmethods which cut down on the total length-of choke cores needed tocbreak the alternate current path or else eliminate such choke cores-altogether;

A furtherobject-isto provide improved circuits and methodswhich reduce both the volt:

agefrom the conductor rolls to ground and'the. current intensitybetweenthe conductor rollsand.

the metal product/at any portion.

A more specific object is to provide circuitsand. methodsv in which. current. is introduced to the. continuousmet'al' product at a minimum of. three.

conductor. rolls, thereby producin two heating zones, thecurrent always. flowing, in oppositedirectionswithrespect to the. direction of. travel of the product .in. the. two zones.

In. accomplishing these and. other objects of.

the invention, I'have proyidedimproyed. details. of'structure, preferredforms ofiwhich are shown.

in the accompanying. drawings, in which:.

Figure 1 is: a schematic view of, a portionof a. processing, line. equipped. with. an. improved heating circuit embodying features of, the. presentinvention and iiigurev 2 is. a schematic. view which shows. amodification.

Figure 1' showsscliematically, a portion of a.

processing line which includes. rolls Ill, l2, l3,,l4

and 15, around which travels a continuous length. metal product S'movingfrom left to right. When.

the product is electrolytically tinned steel strip, roll 4 is situatedin a quench tank It". The heating circuit; hereinafter describedv heats. the strip so that itscoating melts just ahead ofsaid tank. The quench then solidifies the coatingbefore the.

strip contacts thenext. roll.

In accordance with the present. invention, three rolls I3, 12' and [4' are conductor. rolls through which current is introduced to the metal, product, roll I .3 is an insulated roll, and

the remainingroll, i5. is conducting andground-- ed, as indicated'at. I l. The rollscan be of any standard or desired construction and consequently are not shown in. detail; The metal product. wraps around the intermediate conductor roll [2 throughan appreciablearc that furnishes more than a single line contact. The portionof the metal'product'between rolls I 3 and I2 canbe considered'an initial heating zone and that be! tween rolls l2" and [4 a final heating zone, although. in. electro-tinnin applications th power input to the. two. zones isbalanced. to achieve a uniform, heating rate throughout. In.Figure 1. the, lengthy oil the final zone is substantially ent circuit the transformers conveniently can be connected with the polarity marks arranged as indicated in this figure. A conductor 22 connects one end of the secondary windings of both transformers to the intermediate conductor roll l2. A conductor 23 connects the other end of the secondary winding of transformer 20 to the conductor roll 10 at the entry end of the initial heating zone and preferably is grounded, as indicated at 24. A conductor 25 connects the other end of the secondary winding of transformer 2| to the conductor roll M at the exit end of the final heating zone. I prefer that the voltages across the secondary windings of the two transformers be in the same ratio as the lengths of the two heating zones. With the voltages thus adjusted, the amperage is the sam in both zones. In Figure 1 the voltage across the secondary windin of transformer 21 would be twice that of transformer 20.

Since the transformers 20 and 21 are in like phase relation, the intermediate conductor roll I2 is always of polarity opposite that of the entry and exit conductor rolls l and I4, and current always flows in opposite directions with respect to travel of the product in the two heating zones. The voltage outputs from the secondary windings are subtractive; that is, the voltage at conductor roll l4 referred to ground equals the voltage across the secondary winding of transformer 2| minus that across the secondary winding of transformer 20. For example, in Figure 1 if the voltages across the secondary windings of transformers 20 and 21 are 50 and 100 volts respectively, the voltage from conductor roll I4 to ground is only 50 volts. The processing line is shown equipped with the usual choke cores 26, but since they need offer only sufficient impedance to stop current at a much smaller voltage than usual, they can be considerably reduced in length. In the present example they need be only half the usual total length.

It is known that current flow between a conductor roll and a length of metal wrapped therearound is concentrated adjacent lines of tangency at which the metal leaves or comes into contact with the roll. At the intermediate conductor roll I2 there are two regions of maximum current flow, one at the line where the strip comes in from the initial heating zone and another where the strip leaves to the final heating zone. Consequently the current which flows between this roll and the metal product S is divided between two such regions, rather than concentrated in only one, and considerably more total current can be applied without exceeding at any one place the current density limit beyond which there is arcing between the product and the roll. The metal product must wrap around roll l2 through an appreciable arc to assure that current flow is properly divided. Likewise the current introduced to the metal product S through each of the rolls [0 and I4 is only a fractional part of the total current applied. Therefore a considerable larger total amount of current can be applied to the metal product without exceeding the aforementioned limit at any one place. Since the circuit makes it possible to introduce more current to the metal without causing arcing, it is possible to operate the line at higher speeds.

It is to be observed that there is the same voltage drop per unit length of product on opposite sides of the intermediate conductor roll l2. Consequently corresponding spots on the metal on opposite sides of this roll are at the same potential and there is no tendency for current to flow between them. I believe this feature may be of benefit in further minimizing the tendency for arcing at this particular roll.

Figure 2 shows a modification in which the two heating zones are of equal length. Another insulated roll I3a, similar to roll 13, is interposed between conductor rolls l0 and 12. The two transformers are connected in the same fashion as in Figure 1, except in this instance the voltages are equal across their secondary windings. The voltage at both the entry and exit conductor rolls in and [4 when referred to ground is zero, since these voltages are equal and are subtractive. Therefore both these conductor rolls ID and [4 can be grounded, as indicated at 24 and 24a, and the choke cores are eliminated altogether. The modification cuts down the amperage of the current introduced to the strip at any one place in the same way as already described for Figure 1.

While two embodiments of my invention have been shown and described, it will be apparent that other adaptations and modifications may be made without departing from the scope of the following claims.

I claim:

1. In a line for processing metal products in continuous lengths and including a plurality of spaced apart rolls around which a metal product travels, at least three of which rolls are electrically conductive, an electric resistance heating circuit comprising two transformers whose windings are connected in like phase relation, conductors connecting one end of the secondary windings of each of said transformers to one of said electrically conductive rolls which is situated intermediate the other two, and conductors connecting the other ends of the secondary wind ings, repectively to the others of said electrically conductive rolls, said rolls being positioned to wrap the metal product around the electrically conductive intermediate roll through an appreciable arc.

2. In a line for processing metal products in continuous lengths and including a plurality of spaced apart rolls around which a metal product travels, at least three of which rolls are electrically conductive and define an initial heating zone and a final heating zone for the metal product, an electric resistance heating circuit comprising two transformers whose windings are connected in like phase relation, the voltage output at the secondary windings of said transformers being in the same ratio as the lengths of said zones, conductors connecting one end of the secondary windings of each of said transformers to one of said electrically conductive rolls which is situated intermediate the other two, and conductors connecting the other ends of the secondary windings respectively to the others of said electrically conductive rolls, said rolls being positioned to wrap the metal product around the electrically conductive intermediate roll through an appreciable arc.

3. In a line for processing metal products in continuous lengths and including a plurality of spaced apart rolls around which a metal product travels, at least three of which rolls are electrically conductive and constitute an entry conductor roll, an intermediate conductor roll and an exit conductor roll, said entry conductor roll and said intermediate conductor defining an initial heating zone for the metal product and said exit conductor roll and said intermediate conaesaess said rolls being positioned to wrap the metal product around said intermediate conductor roll through an appreciable arc.

GEORGE H. RENDEL.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,718,806 Witting June 25, 1929 2,457,870 Cook Jan. 4, 1949 

